The electromagnetic energy or “waves” transduced by an antenna to or from free space is or are characterized by “polarization.” The free-space form of electromagnetic energy is “elliptically” polarized. A special form of elliptical polarization is termed “linear” or “circular” polarization. In linear polarization, the electric field (E) vector of the radiation remains fixed at a particular orientation relative to the environment over a complete cycle of the electromagnetic wave. The elliptical polarization can be consider as superposition of two mutual orthogonal components of linear polarization simultaneously coexisting and having in common case different magnitude and phase shift. These two components are often referred to as “Vertical” (V) or “Horizontal,” (H) regardless of the actual orientation of the electric field vector relative to local vertical or horizontal. A special form of elliptical polarization is termed “circular” polarization and formed if these two mutual orthogonal linear components have equal magnitude and +−90° phase shift. In circular polarization, the electric field vector rotates about the direction of propagation once during each cycle of the electromagnetic wave, so that its projection onto a plane appears to “rotate.” The direction of rotation of the electric field vector defines the left or right “hand” of circularity and defined the sign of 90-degrees phase shift. The antenna designer will ordinarily design his antenna to respond to either one (V or H) linear or both simultaneously.
U.S. Pat. No. 4,551,692, issued Nov. 5, 1985 in the name of Smith indicates that radar systems presently used frequently employ polarized microwave radiation for surveillance and to detect and track selected target objects. Such radar systems are subject to considerable undesired signal return from raindrops, causing clutter which tends to obscure the desired signals. This effect is particularly pronounced in the millimeter wavelength region because the dimensions of raindrops are approximately equal to the wavelength of the radiation. When circularly polarized microwave radiation is transmitted, the raindrops reflect an opposite sense of the transmitted circular polarization, which is then rejected by the radar antenna and specialized circuitry. The target reflects in the same sense of circular polarization as that transmitted, thereby permitting its direct observation unobscured by rain clutter. The forms of polarized microwave radiation most conveniently generated according to the design of radar antennas and feeds are linear forms of polarization. This has motivated the development of polarizer gratings effective for transforming linearly polarized microwave radiation to a circular form, and for transforming the return signal back to linear form upon return from a target region.
U.S. Pat. No. 7,564,419, issued Jul. 21, 2009 in the name of Patel describes a composite polarizer including a first polarizer having a plurality of metal vanes and also including a second polarizer having a plurality of parallel layers of dielectric material. The first and second polarizers are disposed along an axis and provide differential phase shifts at frequencies f1 and f2. A total of the first differential phase shifts is about 90°, and a total of the second differential phase shifts is also about 90°. The result is that relative rotation of the polarizers allows linear polarization to pass, or allowing conversion of between linear and elliptical polarization and selection of right- or left-handedness for elliptical and circular polarization. The main problem of all polarizers with metal inclusions (vanes, meander lines, etc.) at millimeter-wave frequencies or higher is high Ohmic loss caused by strong skin effect.
Improved polarizers are desired.